1. Field of the Invention
The present invention relates to a nitride crystal and a method for producing the same.
2. Description of the Related Art
Gallium nitride (GaN), which is a wide band gap semiconductor, is known as a material that emits light with a short wavelength such as blue light or ultraviolet light radiation. Furthermore, since GaN has high thermal conductivity and high breakdown strength, it is expected that GaN can realize electronic devices for high-frequency and high power electronic devices that are difficult to realize such as silicon (Si) or gallium arsenide (GaAs) is used.
In regard to the production of GaN substrates, GaN substrates having a diameter (φ) of about 2 inches have been produced by growing a thick film of GaN by a halide vapor phase epitaxy (HVPE) method on a base substrate made of a different material, such as a sapphire substrate or a GaAs substrate, and then by separating the thick film of GaN from the base substrate. However, since the HVPE method involves heterogeneous epitaxial growth of a GaN crystal on a base substrate made of a different material, a difference in the thermal expansion coefficient or lattice mismatch between the GaN and the base substrate is difficult to prevent. Accordingly, the dislocation density of GaN produced by a HVPE method is as high as about 106 cm−2, and there is a strain occurs in GaN due to a difference in thermal expansion between GaN and the substrate. Therefore, a further quality improvement of GaN substrates is desired.
Furthermore, the size of GaN substrates that is required for use in electronic devices is 4 inches or larger in diameter; and increase of the substrate size as well as uniformity of quality among substrates are desired. Moreover, non-polar GaN substrates are considered desirable as the GaN substrates for optical device applications, for the purpose of suppressing the influence of piezoelectric polarization that occurs concomitantly with an increase in the applied voltage. In order to solve the problems described above, it is necessary to develop a high quality bulk GaN crystal.
One method that has been researched and developed to grow a high quality bulk GaN crystal is a flux method in which nitrogen is dissolved in a mixed molten liquid of sodium (Na) and gallium (Ga); and GaN is crystallized and grown therefrom. The flux method is capable of growing a crystal at relatively low temperatures such as 700° C. to 900° C., and the pressure inside the vessel is also relatively low to a level of merely 100 kg/cm2. Thus, the flux method is a practical method for crystal growth.
An article, published in Chemistry of Materials, Vol. 9, 413-416 (1997), reports an example of growing a GaN crystal using sodium azide (NaN3) and Ga as source materials, whereby the source materials are placed and sealed together with nitrogen in a reaction vessel made of stainless steel; and the reaction vessel is maintained at a temperature of 600° C. to 800° C. for 24 to 100 hours.
Japanese Patent Application Laid-open No. 2008-94704 discloses a method for producing a large-sized crystal of GaN by a flux method, in which a needle-like crystal of aluminum nitride (AlN) is used as a seed crystal to grow a columnar crystal of GaN. Furthermore, Japanese Patent Application Laid-open No. 2006-45047 discloses a method for producing a needle-like crystal of AlN that is used as a seed crystal. As such, production of a large-sized crystal by performing crystal growth of a seed crystal by a flux method is a well-known technology.
However, GaN wafers that are obtainable by processing the GaN crystals obtained as described above are not easily visible, because the GaN crystals are colorless transparent crystals, and particularly when the place where a GaN wafer is to be mounted is either white in color or transparent, there is a flaw that it is difficult to find out the exact location of the wafer.
In this regard, Japanese Patent Application Laid-open No. 2002-356398 discloses a technology of making it easier to recognize the contour of the wafer by chamfering the circumferential edge of a GaN substrate that has been shaped into a wafer, and thereby making light diffusely reflected thereon. According to Japanese Patent Application Laid-open No. 2002-356398, the crystal orientation of a GaN crystal can be conveniently identified by forming an orientation flat (OF) on the {10-10} planes, which are cleavage planes of the GaN crystal.
However, in the related art technologies disclosed in Japanese Patent Application Laid-open No. 2002-356398 and the like, there is a need for further processing of GaN substrates such as chamfering or OF formation. Because GaN crystals are hard, fragments (chippings) are likely to be generated during cutting and polishing, and there is a risk that the chippings thus generated may serve as starting points for the breakage of the wafer. Also, in the case of forming an orientation flat (OF) or an interface (IF), there is a flaw in that the amount of loss of the material needed to cut away some parts of the wafer is large.